Fire method and apparatus



Oct. 29, 1957 P. J. CADE ETAL -FIRE METHOD AND APPARATUS 4 Sheets-Sheet 1 Filed May 19, 1951 Tm. Tm. Tm

//v VENTORS P. .1. 0A DE 0. J. MAGDOUGAL 1.

' BY 4576M A T TOHNE Y 1957 P. J. CADE ETAL FIRE METHOD AND APPARATUS 4 Sheets-Sheet 2 Filed May 19, 1951 D. J. MAODOUGALI.

/NVENTO/?S P. J. GADE ATTORNEY Oct. 29, 1957 P. J. CADE ETAL FIRE METHOD AND APPARATUS 4 Sheets-Sheet 3 Filed May 19, 1951 INVENTORS P. J. OADE D.J.MA6DOU6ALL ATTORNEY- Oct. 29, 1957 P. J. CADE ETAL 2,811,711

FIRE METHOD AND APPARATUS Filed May 19, 1951 4 Sheets-Sheet 4 F/ a. 8 LL I46 INVENTORS- BY 4.76m ATTORNEY United States Patent '0 2,811,711 FIRE METHOD AND APPARATUS Phillip J. Cade, Winchester, and Donald J. MacDougall,

Framingham, Mass., assignors to Electronics Corporation of America, a corporation of Massachusetts Application May 19, 1951, Serial No. 227,166

27 Claims. (Cl. 340227) This is a continuation in-part of applicants copending application, Serial No. 211,778, filed February 19, 1951.

This invention relates to improved methods and apparatus for detecting and/or supervising fires. In particular, the methods and apparatus of this invention utilize to advantage certain unique properties of fire flame hereinafter described in detail.

Detection apparatus heretofore used for the protection of life and property from fire have been numerous and varied as to the mode of operation. Most of these devices, however, have the basic shortcoming that they are not fundamentally fire detectors, but rather detectors of some secondary effect of fire, such as temperature change, increase in average light intensity, the'presence of smoke, or the burning or melting action of the flame itself. These devices are, therefore, susceptible to erroneous operation when these effects occur for reasons other than the presence of fire.

For example, thermally-responsive fire detectors are particularly subject to false operation when required to operate under conditions of widely varying ambient temperature, unless their critical operating temperature is set at a high level which usually reduces their usefulness as safety devices. The major defect of these devices, however, is that they are not instantly responsive to fire but actually require the fire to make considerable headway before the necessary thermal level to actuate the detectors can be attained.

Fire detectors which are responsive to average light intensity can be made to operate falsely when subjected to intense light energy from artificial and natural sources. Likewise, smoke from controlled combustion sources will actuate smoke-responsive fire detectors, because these detectors are not actually responsive to the presence of flame in a prohibited area. Smoke detectors are also subject to an objectionable time delay before s'ufiic-ient smoke density can be attained at a detection point to actuate an alarm.

From a comparative aspect, the most satisfactory way to detect fire is by monitoring a volume for the presence of flame therein. This should be accomplished by apparatus which responds instantly and exclusively to fire flame. Furthermore, if the detecting apparatus for any reason loses its capability to perform the aforementioned features of operation, a visual or audible alarm should be immediately given so that the necessary repairs can be made.

Accordingly, it is an object of this invention to instantly and accurately detect fires without the occurrence of false fire alarms.

Another object is to improve the operative reliability of fire detection apparatus whereby the instant the detection apparatus is incapable of fully performing its prescribed functions, an alarm is given.

Another object is to minimize the frequency of the periodic maintenance checks heretofore required to insure proper operation of the detection apparatus,

ice

Another object is to minimize the replacement and maintenance of fire detection apparatus by improvements which make it unnecessary to replace or repair, in the usual case, any of the apparatus components after a delected fire.

Another object is to improve fire detection apparatus whereby, after an initial fire, a simple manual resetting operation will immediately place the apparatus in a condition to detect a second fire.

Another object is to generally improve fire detection apparatus by diminishing the size and weight thereof and reducing the cost of manufacturing the aparatus.

Other objects of two related embodiments of this invention, principally concerned with the detection of both the initiation and the termination of unintentional fires and the supervision of the continuance of intentional fires, are set forth in the description which follows hereinafter.

To construct apparatus which is exclusively responsive to fire flame necessitates a finding of one or more physical properties which is unique and common to the flames of all combustible matter. A frequency analysis of the flame radiation of combustible matter will disclose that the radiation intensity therefrom in all cases is amplitude modulated. This modulated energy, which includes visible and invisible light, comprises many frequency components principally in the range of 0 to 500 cycles per second. Generally speaking the radiation components of the greatest amplitude are concentrated in the lower frequency portions of this range. Sufiicient radiant energy with modulating frequencies of 5 to 25 cycles per second emanates from the flames of all combustible matter to permit use of this property in the design of the novel fire detecting and supervising apparatus of this invention whereby exclusive fire flame response is attained.

Modulated radiant energy is rarely found in nature or in man-made light sources having components in the 5 to 25 cycle frequency range of sufficient intensity to invalidate the practical uniqueness of this property. For example, the visible and invisible light energy reaching the earth from the sun is substantially constant in intensity and has negligible components in the 5 to 25 cycles per second frequency range. Artificial lighting produced by the 25 or 60 cycles per second energies typical of commercial power distribution systems is modu' lated principally at frequencies of 50 or cycles per second, respectively. The frequency of the lighting transients produced by manual operation of lighting switches, or, for example, the passing of the beam of automobile headlights through a window, is usually of the order of one or two cycles per second.

The detecting element of this invention, which searches or sees the actual flame, preferably comprises an electrical photo-responsive device which is responsive to both the visible and invisible light components emanating from fire flame, thereby improving the sensitivity of the resulting fire detection. The electrical output of one or more of these detecting elements is, preferably, connected to a band pass amplifier, which, for all practical purposes, amplifies in a frequency range of approximately '5 to 25 cycles per second. Currents outside this frequency range are attenuated by the amplifier. The output of this amplifier is connected to a limiter so that all the output impulses from the amplifier are confined as to amplitude and translated to a substantially square wave form. These amplitude-limited impulses are then transmitted to a discriminating circuit which is responsive to the summation of a closely spaced impulse train from the limiter. This discriminating circuit serves to implement the filtering of unwanted frequencies by imposing the requirement that at least five impulses must be received from the limiter within a time interval of one second before sutncient potential can be developed by the discriminator to actuate the apparatus which controlsthe fire alarm. This feature of the invention also conclusively assures that the fire alarm will not be operated by momentary accidental fluctuations in illumination as might occur with rapid on and off switching of lights, inasmuch as the frequency of these fluctuations, when computed with respect to the time fraction in which they occur, in some cases actually exceeds the lower frequency limit of the band pass amplifier.

A novel self-monitoring feature of this invention con-- tinually supervises the ability of the fire detection apparatus to fulfill its prescribed functions. This feature obviates the periodic checking of the operability of fire detection apparatus heretofore required. The monitoring apparatus utilizes an oscillator which continually transmits a check signal comprising a continual train of impulses throughout all the detector elements. This signal is, in turn, continually transmitted through the aforedescribcd band pass amplifier, limiter, and discriminator, and also the apparatus which actuates the fire alarm. If for any reasonthis signal is not thusly transmitted continuously and without interruption, a positive visual or auditory alarm is given indicating that the de tecting apparatus is incapable of performing its prescribed functions. Prompt correction of the defect will restore the detecting apparatus to proper operating condition. The fire alarm is not actuated by the self-monitoring signal because the fundamental frequency thereof is limited to approximately one cycle per second, and, therefore, cannot be integrated sufficiently by the discriminator circuit to produce an output potential large enough to actuate the fire alarm.

A second embodiment of this invention is disclosed herein which is particularly adapted to the detection and supervision of unintentional fires in vehicles, particularly in aircraft; whereas the embodiment of the invention previously described is preferably used in the detection of unintentional fires in buildings and other accessible areas, although it may be used to full advantage to supervise certain types of vehicle compartments in which only knowledge of the initiation of a fire is necessary and desirable.

A principal object of the second embodiment of the invention is to improve the supervision of fires in vehicles whereby both the occurrence and the termination of fires therein can be instantly and accurately determined.

Another object is to improve the operative reliability of fire detecting and supervising apparatus for vehicles whereby the capability of the apparatus to fully perform its prescribed functions can be easily and quickly monitored prior to or during vehicle operation.

Another object is to minimize the replacement and maintenance of fire detecting and supervising apparatus for vehicles by improvements which make it unnecessary to replace or repair, in the usual case, any of the apparatus components after a detected fire.

Another object is to provide fire detecting and supervising apparatus for vehicles which is relatively small in size, light in weight and low in cost.

Other specific objects of the second embodiment of the invention will appear from the description which follows hereinafter.

While the principles of operation of this embodiment of the invention and the structure for practicing same are generally applicable to vehicles of all types, the description which follows hereinafter will be particularly concerned with the detection and supervision of fires in aircraft because of the particular vulnerability of this type the weight and frontal area of the power plant, the use of light metals which have poor heat resistance, and improvement in the volatility of fuels, necessarily increase the damage caused by fire in aircraft. High altitude flying has introduced new hazards, such as arcing in electrical systems, and inadequacy of cooling in air-cooled engines, and has given rise to the requirement for supercharging and oxygen supply systems which are in themselves potential sources of fire.

The modern airplane is a complex assembly often powered by several engines, and equipped with a vast num ber of intricate electrical, hydraulic and mechanical devices for performing the various indicating, control, and communication functions necessary for operation and navigation of the craft. Stringent space requirements dictate that large quantities of high octane gasoline be carried in wing tanks, in proximity to the engine nacelles. While considerable research and effort has been directed toward minimizing fire hazards in the design and arrangement of electrical and hydraulic systems and toward the development of fireproof fuel tanks, it is inevitable that increasing complexity of any mechanism which utilizes highly combustible fuels should result in an increasing probability of fire. The hazard is aggravated in the case of the military airplane by the carrying of explosives and exposure to enemy gunfire.

As a result of extensive investigation by military and civilian authorities, both in this country and abroad, it has been determined that the fires most disastrous and difficult to control, that is, fires occurring in flight, usually start in the engine, and that such fires can be extinguished in flight without serious damage to the airplane if the pilot is warned of the. fire and applies the proper firefighting measures in time. The need for quick action and especially for quick detection has been dramatically illustrated in several known instances where fire in an engine nacelle has caused wing failure in less than a minute after the fire started and before the pilot was aware that his engine was on fire. In short, one of the major conclusions to be drawn from such investigations is that the virtual elimination of serious accidents due to fire in flight, with the resulting incalculable saving in lives and property, can be achieved, and at present requires only the development of a reliable alarm system capable of instantly detecting the outbreak of fire. In such a device, reliability is as important as quick action. The usual fire fighting procedures involve shutting down the engine, which may necessitate a forced landing. It can be readily appreciated that one or two such experiences as a result of false alarms may lead pilots to disregard the fire warning system.

Experiments have been made with a variety of fire alarm devices and several types are currently used on aircraft. The detecting elements may be generally classified as thermocouples, thermal expansion switches, and fusible elements. The fundamental disadvantage of such elements for fire detection purposes is that they are temperature-responsive, rather than flame-responsive, and are therefore subject to an inherent time delay which may vary considerably depending on the distance of the element from the flame and on local conditions alfecting heat transmission. To insure reasonably quick operation, temperature-responsive elements must be placed in the engine so as to be downwind from and fairly close to the possible sources of fire, and, for this reason, the elements themselves are usually destroyed when fire occurs. Care must be taken, also, to insure that the detecting elements are not so located as to be cooled by air currents, as, under suchv conditions, these devices have failed to give warning of a serious fire burning only a short distance away. This difficulty can be minimized only by exhaustive experimentation with each new type of engine installation to determine the proper location of detecting elements, and the use of a large number of detecting elements. The thermocouple types of detectors and some of the thermal expanagar-1am Being essentially mechanical switching devices, they are susceptible to vibration and acceleration. The

fusible type is additionally objectionable because it is destroyed on operation and must be replaced, and does not permit checking ofthe system. In order to provide anything approaching complete fire protection, a large number of such elements, spaced not more than a few inches apart, would be required all around each engine. An alternative, the continuous strip element of the fusible type, is now considered by aircraft authorities to be obsolete because of many inherentdrawbacks in its use. Since the weight of the elements themselves and the wiring for such complete protection would be prohibitive, the present fire detection systems on aircraft necessarily represent a compromise between weight and safety considerations.

Furthermore, the development of .jet propulsion for aircraft has given rise-to a fire detection problem for which the thermally operated systems are wholly inadequate.

In comparison to a reciprocating engine of comparable power, the potential danger area of a.jet engine is considerably greater, and the normal ambient temperature in the engine nacelle is considerably higher. Moreover, excessive cooling of the jetpipe is to beavoided, as efficiency is dependent on maintaining a. jet temperature as high as the internal metal parts will withstand. Insulation between the engine and the surrounding structure is ordinarily provided only by a relatively narrow air gap between the engine itself and a conical metal shroud. It is apparent that, with such an arrangement, the occurrence of fire, upon failure of a high-pressure fuel line or turbine blade, for example, is likely to be so rapid and localized that even very closely spaced thermal detectors would'fail to respond in time to prevent severe damage to the airplane. Difficulty from false alarms are also encountered because of the rapid temperature fluctuations and the small differential between normal operating and excessive temperatures.

The fire apparatus of thesecond embodiment of this invention, which is particularly adapted-for use in aircraft, further exploits the flame modulating characteristics of fire flame hereinbefore set forth whereby many of the disadvantages of conventional aircraft fire detectors-are substantially eliminated. As Was previously disclosed, a frequency analysis of the radiant energy emanating from the flames of combustible matter will'reveal the radiation intensity therefrom is, in all cases, amplitude modulated.

As was further disclosed, if a unique range of fire flame K modulating frequencies exists for aparticular installation, apparatus which is responsive only to this range can be constructed to provide exclusive fireflame response.

For example, with respect to the reciprocating engine type of aircraft, it has been found that light components within the range of 5 to 25 cycles per second are rarely produced by sources other than fire flame. The lowest frequency components of modulated light are usually those produced by the chopped light generated by the'low frequency flapping of a loose cowling of an aircraft engine nacelle, or some other similar occurrence. These components have been found to be of theorder of one or two cycles per second. The frequency of the light reflected from a propeller coupled to an idling aircraft engine usually prescribes the'upper limit of the modulated energy unique to fire flame. This frequency has been found to be well above 25 cycles per second.

With respect to jet type aircraft, -the lower modulating frequencies produced by non-flame sources are substantially the same as those of the reciprocating engine type of aircraft, and the upper limit of frequencies is usually determined by the artificial lighting produced by the'relatively high power supply frequencies typical of aircraft power plants. This artificial lighting is amplitude modulated at twice the supply 'currentf'requencies and is Well above 25 cycles per second.

fh'e embodiment of the fire apparatus of this invention which wasparticularly constructed for-aircraft differs generally in the following aspects from the embodiment which was particularly constructed for fire detection in buildings or other accessible locations.

The fire apparatus for aircraft comprises a novel'latching feature whereby the extinguishment of a fire within,

for example, an aircraft engine nacelle, can be reliably indicated by the apparatus. A visual inspection to ascertain the continuance or extinguishment of a fire is not practical in aircraft because, among other reasons, the fire may be wholly confined within an enclosed compartment or a portion of the aircraft not easily supervised by visual inspection. Knowledge of the progress of a detected fire within aircraft is essential so that the pilotcan intelligently plan the necessary course of action. For example,'if a fire Within an aircraft nacelle persists, the pilot would possibly attempt a forced landing, but if such a fire is extinguished for one reason or another, a forced landing :probably would not be attempted. This feature of opera- .tion is not incorporated in the first embodiment of this invention because in fire warning systems for buildings :and .other accessible structures, including certain vehicle compartments, the continuance of the fire alarm until a manual alarm-removing operation takes place is desirable sothat attention will be brought to the fact that a fire has occurred.

A second difference between the two embodiments of the invention appears in the testing arrangement utilized to supervise the capability of the apparatus to perform their fire detecting functions. The testing arrangement for the first embodiment continuously transmits a check signal comprising a continual train of impulses throughout the apparatus as previously described. The testing arrangement for vehicle fire apparatus as disclosed in the second embodiment operates only in response to the manual operation of a test switch which checks one fire detector channel at a time. The specific reasons for this structural difference will appear from the detailed description which appears hereinafter.

A third embodiment of the invention is disclosed herein which is particularly adapted to the supervision of intentional fires.

The principal object of this embodiment of the invention is to provide a completely reliable and instanstaneous indication of the presence and the termination of an intentional or wanted fire. This type of apparatus can be used, for example, to supervise the continued existence of flame in the burner of jet aircraft or the continued existence of flame in an oven or the like. This apparatus can also be used to produce a control function responsive to the termination of a fire.

This embodiment of the invention differs from the first two embodiments described in that the. circuit structure is economically designed to produce the requisite reverse mode of operation. That is, output indications are to be provided in the absence of flame and not in response to the initiation or detection of a fire. Broadly speaking, this reverse function is accomplished with a band pass amplifier, the output of which is filtered to produce a direct-current bias. This bias is removed from the power tube which controls the operation of the output circuit of the apparatus whenever the monitored fire flame goes out for any reason.

In order that all of the features of this invention and the mode of operation thereof may be readily understood, a detailed description follows hereinafter with particular reference being made to the drawings, wherein:

Fig. 1 is a schematic circuit diagram of the first embodiment of the fire detection apparatus of this invention;

Fig. 2 is a perspective view of a photo-responsive fire detection element suitable for use in the circuit of Fig. 1;

Fig. 3 is a fire detection element assembly suitable for housing the element of Fig. 2;

Fig. 4 is. a schematic diagram of the second embodiment of the fire apparatus of this invention which is particularly adapted for use in aircraft;

Fig. 5 is a simplified perspective view of a fire detector pick-up unit suitable for use in the circuit of Fig. 4;

Fig. 6 is a simplified showing of the installation of the fire apparatus of the second embodiment of this invention, together with a plurality of pick-up units, in a et aircraft;

Fig. 7 is a simplified showing of the installation of the fire apparatus of the second embodiment of this invention, together with a plurality of pick-up units, in a reciprocating engine type aircraft; and

Fig. 8 is a schematic circuit diagram of the third embodiment of this invention which is suitable for supervising intentional fires.

Referring now to Fig. 1, fire detection elements 1, 2 and 3 are serially connected between terminals 4 and 5. These detection elements are preferably photoconductive cells which are sensitive to both visible and invisible light, such as, for example, lead sulfide photoconductive cells. In actuality, these elements are usually disposed at a distance from one another on the walls or ceilings of a building under fire supervision. The series impedance presented by elements 1, 2 and 3 to terminals 4 and 5 varies inversely to the amount of cell-responsive radiant energy impinging upon the elements. The maximum number of fire detection elements which can be connected in series depends, generally, upon the amplification of the apparatus which is to utilize the impulses therefrom. In the particular preferred embodiment of this invention shown in Fig. 1, one to six elements connected between terminals 4 and 5 have operated satisfactorily.

A positive breakdown potential is applied to electrode A2 of gas diode T2 through resistor 20. serially-connected capacitor 16 and resistors 9 and 10 are connected across gas diode tube T2. Components 9, 10, 16, and T2 comprise a relaxation oscillator which continually transmits a one cycle per second self-monitoring signal throughout the fire detection apparatus. The output signal of this oscillator developed across resistor 9 is applied in series with the potential between terminals 4 and 5 to terminal 4 and ground.

The positive energizing potential for detection elements 1, 2 and 3 is applied through load resistor 7. Resister 12 and capacitor 13 comprise an RC filter section for minimizing the alternating current components of the positive potential applied to eicments l, 2 and 3 through resistor 7 and to diode T2 through resistor 20.

A high voltage positive potential is applied to conductor 21 by conductor 3il'from power supply output terminal 39. The power supply includes transformer 50, which has a primary winding connected to alternating-current supply terminals 52 and 53, a center-tapped step-up secondary winding 49, and a step-down filament winding 51. Anodcs A10 and P10 of full-wave rectifier tube T10 are connected directly to the end terminals of secondary winding 49. The center tap of secondary winding 49 is connected directly to ground. Cathode C10 is connected directly to a filter section having 21 capacitor 47 input between cathode C and ground, and an inductor 45 connected to the positive potential terminal of capacitor 47. Voltage regulator tube T7 and its associated limiting resistor 42 are connected directly between ground and the left terminal of inductor 45. The regulated output voltage of the power supply appears between terminal 39 and ground.

Secondary winding 51 energizes parallel-connectcd filaments F1, F3, F4, F5, F6 and F9. This winding also energizes filament P10 in a circuit which includes relay winding I.

The positive potential on conductor 21 is applied to anode A through load resistor 18. Control grid G1 is connected to terminal 4 by serially-connected resistor 11 and capacitor 6. The common junction of resistor 11 and capacitor 6 is connected to ground through resistor 3. The T section, comprising serially-connected capacitors 15 and 17 connected between control grid G1 and anode A1, and resistor 14 connected between the junction of said capacitors and ground, is a high-pass degenerative feedback network for tube T1, whereby currents of frequencies greater than 25 cycles per second are attenuated by the T1 amplifier stage. Low-frequency components below 5 cycles per second applied between terminal 4 and ground are attenuated by the L section comprising capacitor 6 and resistor 8. Resistor 11 isolates the output of the degenerative feedback network at the lower terminal of capacitor 15 from the relatively low impedance of detecting elements 1, 2 and 3, so that the feedback potential applied to control grid G1 will not be excessively attenuated.

The output signal potential developed across load resister 13 by tube T1 is coupled to control grid G3 of tube T3 by serially-connected capacitor 19 and resistor 22. A positive potential from conductor 21 is applied to anode A3 through load resistor 27. The T section, comprising serially-connected capacitors 24 and 26 connected between control grid G3 and anode A3, and resistor 23 connected between the junction of said capacitors and ground, is a high-pass degenerative feedback network for tube T3 whereby currents of frequencies greater than 25 cycles per second are further attenuated by the amplifier stage comprising tube T3 and its associated components. Low-frequency components of less than 5 cycles per second appearing between the lower terminal of resistor 18 and ground are attenuated by the L section comprising capacitor 19 and serially-connected resistors 22 and 25. Resistor 22 isolates the output of the degenerative feedbackv network appearing at the lower terminal of capacitor 24 from the relatively low impedance of the lower terminal of resistor 13 with respect to ground so that the feedback potential applied to control grid G3 will not be excessively attenuated.

Tubes T1 and T3 and their associated components, therefore, generally comprise a band pass amplifier for amplifying potentials applied between terminal 4 and ground and having a frequency in the range of 5 to 25 cycles per second. All other frequencies generated by dctecting elements 1, 2 and 3 are relatively attenuated by the band pass amplifier stages.

The output signal of the band pass amplifier which is developed across load resistor 27 is coupled to control grid Gs by serially-connected capacitor 28 and resistor 30. GIld return resistor 29 is connected between ground and the junction of capacitor 28 and resistor 30. Tube T4 is biased to a negative value greater than cut-off by the potential drop created across cathode resistor 32 by current supplied through resistor 34 from high voltage terminal 39. A positive potential is applied to anode A4 through load resistor 31.

Tube T4 and its associated components provide grid and plate limiting so that only negatively directed im pulses of substantially the same amplitude and substantially square wave form appear between the lower terminal of resistor 31 and ground in response to both positive and negative impulses of varying amplitudes applied to the input of tube T; by the output of the band pass amplifier. That is, negative impulses applied to the input of tube T4 do not develop a positively directed impulse between the lower terminal of resistor 31 and ground bccause tube T4 is biased to a value exceeding cut-off by the biasing potential of cathode resistor 32. Positive impulses applied to the input of tube T4, which exceed the cut-off bias value, develop negatively directed impulses of constant amplitude between the lower terminal of resistor 31 and ground because of the limiting action of gridcurrent flow through resistor 30 and saturation platecurrent flow through resistor 31.

The substantially square negative impulses developed across load resistor 31 are applied 'to'the differentiation to one another, so that tube T5 operates as'a'diode. The space path of this diode shunts resistor 35 directly. Therefore, the positive impulses generated by differentiation in capacitor 33 and applied to tube T5 are shorted to ground by the low impedance presented thereto by tube T5. However, the negative impulses generated by differentiation in capacitor 33 and appliedto tube T5 are not shorted to ground becauseof the high impedance presented thereto by tube T5. Accordingly, these negative impulses charge capacitor 37 through resistor 36. Any charge on capacitor 37 discharges through resistors 35 and 36. The time constant of the charging path for capacitor 37 is shorter than the discharge time constant. Therefore, the greater the number of negative impulses applied to the inputof tube T5 withina specified time interval, the greater the potential developed across inte-' grating capacitor 37. The differentiation, rectification and integrating components together comprise a low-frequency discrimination network.

In the preferred embodiment of Fig. 1, the component valuesatfectin-g the time-constant for the charge and discharge circuits for capacitor 37 are selected so that a specified negative potential value will appear across ca- :pacitor 37 when five or more negative impulses are applied to tube T5 within an-interval of one second. 'When this value of negative potential is applied'to control grid G6, the plate current thereof is kept to 'a-valuebelow the operate and release values for fire relay FR.

Before energization of trouble relay TR and fire relay FR, cathode C6 is connected directly to ground by a path which includes the break contact of the continuity transfer of trouble relay TR. After energization of fire relay FR and trouble relay TR, cathode C6 is connected directly to ground by a path which includes the make contact of relay FR and the make contact of the continuity transfer of relay TR.

If the potential across capacitor -37 is sufficiently negative to keep the anode A6 current flowing throughrelay FR below the operate or hold relay values, as thecase may be, the break contact of relay FR is closed. Ifthe upper make contact of trouble relay TR is also closed and double-pole double-throw switch S is in the upper or operate position shown in the drawing, audiblefire alarm FA is energized by current flow from the secondary of transformer 48 ina circuit which includes'the break contact on relay FR, the upper make contact on relay TR, fire alarm FA, and the upper pole of switch S, back to the secondary winding of transformer 48.

Operation of the fire alarm FA, therefore, requires fire relay FR be released, trouble relay TR be operated, and switch S be in the operate position shown in the drawing.

The amplitude of the plate-current flow in the anode A9- cathode C9 space path of tube T9 controls the operation of trouble relay TR. This currentjfiows from the positive terminal 39 of the power supply throughconductor 38, conductor 21, the secondary winding of transformer 48, the upper pole of switch S when positioned in the operate position, fire alarm FA, relay TR, anode Ag-cathode C9 space path, resistor 46, make contact on relay FR, and the break contact of the continuity transfer of relay TR to ground. Operation of relay FR is, therefore, required to initially actuate relay TR. After relay TR is actuated, resistor 46 is grounded throught he make contact of the continuity transfer of relay TR and operation of relay FR is no longer required. Tube Ta is normally biased to cut-off 'by'the potential drop across resistor 46 created by current flow through resistor '44 from power supply terminal 39.

Resistor 43 interconnects the junction of serially-connected resistors 40 and 41. Diode Ta connectsanode As directly to the upper terminal of resistor -40 so that the so that current flow from battery B energizes audible and/ or visual trouble alarm TA. The energizing circuit for the trouble alarm TA is completed wheneverthe apparatus of Fig. 1 is incapable of performing its prescribed function. If, for example, the filament supply voltage should fail, the break contact on relay 1 closes, grounding control grid G9, thereby causing relay TR to release the break contact thereon and to complete the trouble alarm circuit. The full measure of the continual trouble supervision provided by the trouble relay TR and its associated trouble alarm circuit will be readily apparent from a portion of the description which follows hereinafter.

The fire detection apparatus of this invention is in operative readiness whenever switch S is placed in the upper position shown in Fig. 1, and audible fire alarm EA will be operated whenever a fire is detected.

If it is desired to fire-test the apparatus without operating the audible alarm, switch S-should be positioned in its lower, or test position. This positioning disconnects the fire alarm FA from the secondary winding of transformer 48 and connects test lamp TL'thereto. This lamp is energized if the break'contact on the'fire relay is closed. If the apparatus responds properly to a test fire, a sufficiently large negative potential appears across capacitor 37 to reduce the current "flow through tube T6 to a value below the hold value for relay FR, thereby 'cuasing the closure of the break contact on relay FR and the consequent lighting of test lamp TL.

Visual and/or audible trouble alarm TA is actuated whenever switch S is in its lower test position, thereby indicating that the fire detection apparatus is not in operative readiness. That is, fire alarm FA cannot be operatedin response to a detected fire. The operation of the trouble alarm during a testing operation is, therefore, a continual reminder that switch S should be returned to its upperor operate position after the test has been completed. This actuation of trouble alarm TA is caused by the grounding of control grid G9 through a circuit which includes the bottom pole of switch'S, thereby releasing trouble relay TR and closing the lower break contact thereon.

Trouble alarm TA is also operated whenever the detecting apparatus is incapable of fully performing its functions. This feature of operation, which will be more fully described later, results in part from the continual self-monitoring output signal generated by the relaxation oscillator comprising gas diode T 2 and its associated components.

The detailed operation of the circuit of Fig. 1 prior to the detection of a fire is as follows: before the application of power to the circuit, switch S is placed in the upper or operate position so that the fire alarm FA will be operated if and when radiant energy from fire flame impinges upon one or more of fire detecting elements 1, 2 or 3. A suitable alternating-current potential is thereafter applied to terminals 52 and 53 to energize the primary windings of transformers 48 and 50. The output potential of secondary winding 51 heats filaments F1, F3, F4, F5, F6 and F9. Filament P10 is also heated by current from secondary winding 50 flowing in a path which includes relay winding 1. Relay I is accordingly energized, thereby opening the break contact'thereon and removing ground from control grid G9.

Full-wave rectifier tube T10 operates in the conventional manner to transform the alternating-current potential from secondary winding 49 to full-wave direct-current impulses appearing between cathode C10 and ground. The filter section comprising capacitor 47 and inductor 45 substantially filters the alternating-current components from the full-wave direct-current impulses applied thereto. The

relatively smooth direct-current'potential appearing between the left-hand terminal of inductor 45 and ground is voltage-regulated by gas diode T1 and its currentlimiting resistor 42. A voltage-regulated positive potential, therefore, appears between terminal 39 and ground.

This positive potential is applied to anode As through conductor 38, conductor 21 and relay winding PR, and ground potential is applied to cathode Cs through the break contact of the continuity transfer on trouble relay TR. In the absence of fire, insufiicient negative bias is applied to control grid G6 to prevent the operation of fire relay FR, because the charge on capacitor 37 is that produced solely by the signal impulses transmitted thereto through band pass amplifier tubes T1 and T3, and limiter tube T4. by the relaxation oscillator comprising components 9, 10, 16, and T2. A detailed explanation of the transmission of these oscillator signals follows hereinafter.

The consequent closure of the make contact on fire relay FR, due to the operation of said relay by the plate current of tube T s, grounds the upper terminal of resistor 46 in a path which includes the make contact of fire relay FR and the break contact of the continuity transfer of trouble relay TR. The positive potential of terminal 39 applied to cathode resistor 46 through resistor 44 biases tube T9 to cut-off. The positive potential of terminal 39 is also applied to anode A9 through a path which includes conductor 38, conductor 21, the secondary winding of transformer 43, fire alarm FA, and trouble relay TR.

This cut-off bias is overcome by a positive signal applied to control grid G9 of tube T9 by the relaxation oscillator output signal appearing across resistor 9. This output signal is applied to terminal 4 through seriallyconnected fire detection elements 1, 2 and 3. The frequency of this signal is approximately one cycle per second, but because of the relative strength thereof, it is transmitted through the band pass amplifier stages comprising tubes T1 and T2, notwithstanding the five cycles per second low-frequency cut-olf of this amplifier. This signal passes through limiter tube T4 and appears across integrating capacitor 37 in sufficient amplitude to apply a negative potential to control grid Gs which reduces the plate-current flow through fire relay FR to a value slightly greater than the release value therefor. This decrease in current flow through relay winding FR raises the potential of anode As of gas diode Ts. Because of the constant voltage characteristics of diode Ts, the potential of cathode C3 is also increased by the same increment. This positive increment is divided by seriallyconnected resistors 40 and 41, and a portion thereof is applied to control grid G9 through resistor 43 in sufficient amplitude to overcome the cut-off bias of tube T9 and cause sufficient current flow in the anode Ara-cathode C9 space path to energize trouble relay TR.

The energization of trouble relay TR transfers ground to cathode Cs from a path, which before energization of trouble relay TR included the make contact of fire relay FR and the break contact of the continuity transfer of. trouble relay TR to a path which includes the make contact of fire relay PR and the make contact of the continuity transfer of trouble relay TR. The operation of fire relay FR is, therefore, unaffected by the operation of trouble relay TR.

The operation of the trouble relay TR opens the lower break contact thereon and prevents battery B from applying an operating current to trouble alarm TA.

The fire alarm FA is not operated by current flow from the secondary winding of transformer 48 because the energizing circuit therefor includes the opened break contact of energized fire relay FR. The plate current for tube To is considerably smaller than the operate value for fire alarm FA.

Thus, when a fire is not detected by any of elements 1, 2 and 3, fire relay FR is actuated, preventing the operation of the fire alarm FA, and the trouble relay TR is 12 also actuated preventing the operation of trouble alarm TA.

Because of the novel mode of operation of trouble relay TR, the capability of the fire detection apparatus to per form its prescribed functions is continually monitored. If the relaxation oscillator output signal across resistor 9 is not continually transmitted through serially-connected fire detection elements 1, 2 and 3, band pass amplifier tubes T1 and T2, limiter tube T4, the discriminating network including tube T5 and integrator capacitor 37, and tube Ts to control grid G9, trouble relay TR will permit closure of its break contact and consequent operation of trouble alarm TA. Operation of the trouble alarm will, therefore, immediately reveal any defective operation in the aforementioned components so that the necessary repairs may be made.

Likewise, if the fire alarm should burn out, or switch S be removed from its operate position, or the secondary winding of transformer 48 be opened, the trouble relay TR will release its lower break contact and operate the trouble alarm TA. A burning out of filament 10 will permit closure of the break contact of relay I and ground control grid G9, thereby de-energizing trouble relay TR and operating trouble alarm TA. Accordingly, the fire detection apparatus of this invention, for all practical purposes, is self-checking so that if at any time it is incapable of detecting a fire and operating a fire alarm in response thereto, a trouble alarm will be given so that the necessary repairs can be made.

Furthermore, a shorting of most of the tube electrodes for tubes T1, T3, T4 and T to their respective filaments will apply a resulting signal to integrating capacitor 37 sufficiently large to release fire relay FR and thereby operate the filre alarm FA. This is because the filament potential of secondary winding 5]. is great enough to transmit a sufficiently large signal to capacitor 37 to simulate a fire. With certain selection of circuit components to assure the release of fire relay FR in response to a tube short, the potential of secondary winding 51 should be much greater than that required for the circuit filaments. In this case the filaments are supplied from an intermediate tap on the filament winding.

The detailed operation of the circuit of Fig. 1 after the detection of fire is as follows: with switch S in the operate position as shown in Fig. l and fire relay FR and trouble relay TR being operated as hereinbefore described, if radiant energy emanating from fire impinges on one or more of fire-detecting elements 1, 2 or 3, a negative potential is developed across integrating capacitor 37 which reduces the plate current for tube T6 to a value less than the hold value for fire relay FR, thereby releasing the contacts thereon. The closure of the break contact on the fire relay completes the energizing circuit for the fire alarm FA, thereby warning ofthe presence of a fire in the area supervised by the detecting elements 1, 2 and 3. The secondary winding of transformer 48 supplies the fire alarm FA energizing current in a circuit which includes the break contact of fire relay FR, the upper make contact of. trouble relay TR, fire alarm FA, the upper section of switch S in the position shown in Fig. l, and a conductor returning to the secondary winding of transformer 48.

The negative potential across capacitor 37 which causes the release of fire relay FR is developed as follows: the radiant energy impinging upon one or more of fire-detecting elements 1, 2 and 3 causes a modulation of the series impedance thereof at the same modulating frequencies present in the fire flame. This impedance variation devclops a voltage drop across load resistor '7 having the same frequency components.- The potential applied to terminals 4 and 5, therefore, is amplitude modulated at the same frequencies as the fire'fiame. The potential across terminals 4 and 5 is applied in series with the relaxation oscillator output signal developed across resistor 9 to terminal 4 and ground. I I

The frequency components in the range of 5 to 25 "theband pass amplifier stages. however, will discriminate against this signal because at cycles per second applied between terminal 4 'andjground are amplified by band pass amplifiertube T1 and its associated amplifier components, thereby developing an amplified signal across load resistor 18. The signal developed across load resistor '18 is coupled by serially-connected capacitor 19 and resistor 22 to-control grid '63 of tube T3, which tube, together with its associated components, comprises a second band pass amplifier stage. The transmitted components are, therefore-further amplilied and they ultimately appear across output load resistor 27. This signal is coupled to control grid G4 of limiter tube T4. by serially-connected capacitor 28 and resistor 30. The negatively directed alternations thereof are limited completely by limiter tube T4 because tube T4 is biased to a value exceeding cut-off. When the potential of the positive alternations of the signal developed across resistor 29 exceeds the bias potential across resistor 32, grid-current flow through resistor 30 and saturation platecurrent flow in resistor 31 limits the negatively directed impulses developed across resistor 31in the conventional manner. Accordingly, only negatively directed impulses of constant amplitude are developed across load resistor 31 .of limiter tube T4.

These negatively directed impulses aredifferentiated by capacitor 33 and resistor 36. Any positive-impulses-which are generated by diiferentiation in capacitor 33 are shorted to ground by the low impedance presented thereto by tube T5. The negative impulses generated by differentiation are not shorted out by 'tube T5, and, "therefore, charge capacitor 37. Because of the selection of component values hereinbefore explained, the charging time constant for capacitor 37 is shorter than its discharge time constant. Therefore, the greater the'nuniber of'impulses applied to capacitor 37 within a given "time interval, the greater the value of negative potentialintegrated. When five impulses per second are applied to capacitor 37, the integrated potential is sufficiently great to reduce the plate current flow in tube Ts below the-hold value for fire relay FR, thereby actuating fire alarm FA as'hereinbefore explained.

The requirement that at'least five impulses be applied to capacitor 37 in a second before suflicient integration occurs, prevents a false alarm from occurringsin response to rapid on and off switching of lights.

It is possible to have two light *cycles generated in an interval of'less than a fifth of a second by this switching operation. A signal will, therefore, be transmitted-through The integrator .circuit,

least five impulses per second are'required to produce sufficient integration to release fire relay FR.

Operation of fire alarm FA may be stopped by moving switch S to its lower or reset position so as to open the fire alarm circuit. 'If the fire has been-extinguished, a repositioning of the switch to its upper or operate position will not complete theenergizing circuit for fire alarm FA, because the break contact of .fire relay FR is opened by the reoperation of fire relay FR. The fire detection apparatus is, therefore, easily placed in readiness to-detect a second fire without replacement or maintenance of components.

If it is desired to test the operation of the apparatus in response to a controlled testing flame without operating the alarm, switch S is positioned to its lower or reset position. This disconnects fire alarnrFA from the secondary winding of transformer 48 and connects test lamp TL thereto. Therefore, when the testing fire .causes closure of the break contact of firev relay FR in a manner heretofore explained, test lamp TL is energized by current flow :from the secondary windingof transformer 48 flowing in a circuit which includes the break .contact'of .fire relay FR, test lamp TL, and the upper .sectionrof switch S, in its lower switch .position, back to the secondary winding of transformer 48. I

It should'be noted that, whenever switch S is positioned .inFig. 1. have corresponding functions are identified with the same in its reset .or test position, control grid G9 is grounded, thereby causing trouble relay TR1to release its lower armature and completeithe energizing circuit for trouble alarm TA. The operation of this alarm is a continual reminder that switch S should be reset after the test to its operate or upper position so that the apparatus will again be able to operate the fire alarm'in response to a detected fire.

There is shown in Fig. 2 a lead sulfide photoconductive cell suitable for use as a fire-detecting element for the circuit of Fig. 1. The cell comprises two aquadag-circular rings 55 and 56 painted on the inner'side of hermetically sealed envelope 54. A lead sulfide layer provides a .photoresponsive electrical path between the aquadag rings. External electrical connections are made to rings 55 and 56 by socket contact to contacts 57 and 58, respectively. This .cell structure is particularly useful in fire-detecting apparatus because it provides fire detection in a hemispherical volume .represented by shell'60. Consequently, very few .of these'cells are required to supervise a large volume for fire. A more detailed disclosure of the structure and operation of the photoconductivity cell shown in Fig. 2 may be found in U. S. Patent No. 2,636,100, 'issued April 21, 1953, to N. Anderson.

The cell of Fig. 2 can be protectively mounted on a wall or ceiling by the housing assembly shown in Fig. 3. This assembly comprises a metallic box 61 which is preferably recessed in wall or ceiling 62, therebyexposing only the 'bottomportion of the assembly, which portion includes the photoresponsive portion of the cell. The glass envelope 54 of the cell is protected by translucent cover 63, which cover should be constructed of .a material that does not appreciably absorb the radiant energy emanated by fire flame. The necessary connections to the cell are made by conductors 64 and 65. The portion of these conductors outside box 61 interconnecting a plurality of cells should preferably be located within a metallic conduit.

A substantial portion of the circuitry of the second embodiment of this invention shown in Fig. 4 is identical to that of Fig. 1. In particular, the band pass amplifier comprising tubes T and T3 and their associated components is the same as the band pass amplifierof Fig. 1. Likewise, the limiter circuit comprising tube T and its associatedcomponents is the same as the limiter circuit shown The circuit components of Figs. 1 and 4 which reference characters, and explanation of the operation thereof can be had by referring to the appropriate portion of the detailed description of the circuit of Fig. 1 hereinbefore set forth.

The input signal to the band pass amplifier comprising tubes T1 and T3 is developed across load resistor 89, which resistor is commonly connected in the anode circuits for mixer tubes T11, T12 and T13. This signal is applied to control grid G1 by serially-connected capacitor 87 and resistor 8S. Anodes A11, A12 and A13 of the mixer tubes are connected in multiple with respect to one another so as to develop a common output across load resistor 89, whereas the inputs to mixertubes T11, T12 and T13 at terminals A, B and C, respectively, are isolated from one another to provide a signal mixing function. Photoconductive cells P P2 and P3 of fire detector pick-up units D1, D2 and D3 are connected to terminal A through isolating resistors 66, 67 and 68, respectively; photoconductive cells P4, P5 and P6 of fire detector pick-up units D4,

D5 and D6 are connected to terminal B through isolating resistors .69, 70 and 71, respectively; and photoconductive cells P7, P8 and P9 of fire detector pick-up units D7, D8 and D9 are connected to terminal C through isolating re- .sistors 72, 73 and 74, respectively. The energizing currents for the three sets of photoconductive cells P1 to P3, P4 to P6, and P1 to P9 are supplied through load resistors 75,76 and 77, respectively. Resistors 78, 79 and 80 aid in keeping the direct-current potential of terminals -A, B and Cat a substantially constant value during the operative life of the circuit apparatus. Coupling capacitor 81 and grid-return resistor 84, coupling capacitor 82 and gridreturn resistor 85, and coupling capacitor 83 and gridreturn resistor 86 provide conventional RC inputs for mixer tubes T11, T12 and T13, respectively.

Each of detector units D1 to D9 comprises a corresponding photoconductive cell P and test lamp L. Each of the photoconductive cells P and its associated test lamp L are preferably located within a housing of the general type shown in Fig. 5, whereby the light energy from lamp L can impinge upon the active surface of photoconductive cell P in response to the operation of test lamp L. Referring in particular to Fig. 5, the assembly thereof comprises a metallic tubular housing 122, a portion of which is broken away so as to show the positioning of photoconductive cell P therein. The photoresponsive area of this cell shown generally at 121 comprises the common area between electrodes 123 and 124. Lamp L is located within shell 125, which shell is screwed to a side portion of housing 122. The light from test lamp 1. passes through narrow slit 126 in shell 125, and a portion of the components thereof are transmitted through filter window 120 so as to strike the active portion of photoconductive cell P at 121. The operation of test lamp L is used to simulate a fire and thereby test the fire apparatus of the circuit of Fig. 4 in a manner hereinafter explained in detail. A complete structural description of a photoelectric cell device similar to the fire detector housing shown in Fig. 5 is disclosed in U. S. Patent No. 2,631,247, issued March 10, 1953, to B. E. Shaw.

Test switch TS comprises nine test positions and a tenth operate position 0. When test switch TS is positioned at one of the test positions, the corresponding test lamp L is energized by the potential applied by secondary winding 112 through interrupter INT. Interrupter INT may be any conventional apparatus for interrupting or otherwise opening the test lamp energizing circuit at a frequency within the operating range of the band pass amplifier comprising tubes T1 and T3. This operating range is, in the case of most aircraft, between the limits of 5 and 25 cycles per second. Interrupter INT may, therefore, have an interrupting frequency of, say, cycles per second.

If test switch TS is positioned at 0, none of test lamps L are energized because there is no ground return path for any of the lamps through secondary winding 112.

Photoconductive cells P1 to P9 are connected in parallel with respect to one another in the circuit of Fig. 4 so as to provide fail-safe fire protection. That is, if one or more of the fire detector pick-up units D is destroyed by fire or rendered inoperative for any reason, the remaining detector units will nonetheless be capable of detecting a fire satisfactorily or of indicating the termination of a detected fire. In the circuit of Fig. l, fire detection elements 1, 2 and 3 are serially connected. Therefore, the failure of one of said units will render the remaining units inoperative. Such an occurrence is not critical in the embodiment of Fig. 1 because the transmission of the check signal of the apparatus of Fig. l is interrupted by failure of one of the serially-connected detection elements. This transmission failure of the check signal will cause the operation of trouble alarm TA, thereby giving an indication of trouble. However, in the apparatus shown in Fig. 4 adapted to aircraft, the termination of tire must be reliably indicated notwithstanding the failure of one or more of fire detector pickup units D. This requires a parallel connection of the detection elements, as distinguished from a series connection. This parallel connection of the fire detection pick-up units also imposes the restriction that the units must be tested individually, as distinguished from a simultaneous testing of the units as was possible in the circuit of Fig. l.

The negatively directed square-shaped impulses developed across load resistor 31 of limiter tube T4, when the appropriate signal frequencies appear across output load resistor 27 of band pass amplifier tube T3, are differentiated principally by capacitor 33 and resistor 35 so that both positive and negative impulses of short duration and constant amplitude appear across resistor 35. Resistor 91 and capacitor 92 comprise a highfrequency filter whereby any components exceeding the upper frequency range of the band pass amplifier and reaching this point of the circuit are further attenuated. The component values for the filter section are selected so as not to substantially affect the wave shape of the difierentiated positive impulses appearing across resistor 35 at a rate of approximately 5 to 10 per second.

The positive impulses developed by diiferentiation in capacitor 33 and resistor 35 are transmitted by tube T14 so as to charge integrating capacitor 93. The negative impulses generated by differentiation do not affect the integration produced by capacitor 93 because of the high back resistance of tube T1 to negative impulses. The charging time constant for capacitor 93 is shorter than its discharge time constant. Therefore, the greater the number of impulses applied to capacitor 93 within a given time interval, the greater the value of positive potential integrated thereacross. When at least five impulses per second are applied to capacitor 93, the integrated potential applied to resistor 96 through resistor 94 is sufficiently large to break down thyratron type gas tube T15.

The anode A15 to cathode C15 breakdown potential for tube T15 is provided by the alternating potential output of secondary winding 106. A negative bias is applied to cathode C15 and control grid G15 through quickheating diode T18 by secondary winding 109 during the filament warm-up period for rectifier tube T17 and gas tube T15. This bias arrangement prevents the operation of fire indicator lamp FI by an impulse developed in the primary winding of output transformer 103 due to breakdown of tube T15 during the filament warm-up period.

After filament F17 of full-wave rectifier tube T17 reaches its operating temperature, the positive impulses at cathode C17 are filtered by the T section comprising resistors 100 and 102 and capacitor 101. Potentiometer 99 is connected directly across the positive terminal of the power supply and ground so that the movable tap thereof can be adjusted to provide a direct-current bias to gas tube T15 whereby the required integration must occur in capacitor 93 before tube T15 is broken down.

Electrostatic shields 104, 107 and 108 prevent the development of an alternating potential across the lower tapped portion of potentiometer 99 by secondary windings 104 and 106. Inasmuch as this portion of potentiometer 99 supplies the negative bias to tube T15 after the filament warm'up period, any appreciable alternating potential appearing across this portion of the potentiometer would tend to fire tube T15 falsely.

Gas diode T16 provides conventional voltage regulation for the power supply, and the filter section comprising resistor 98 and capacitor 97 provides additional filtering for the current supplied to photoconductive cells P1 to P9. A tap on secondary winding 109 supplies the appropriate filament potential for the tubes having filaments. If tubes are selected for use in this circuit having diiierent filament values, the necessary voltage taps must be made to secondary winding 109.

The detection and the supervision of fire by the circuit of Fig. 4 is as follows. Test switch TS should be positioned at operate position 0 before the application of the operating potentials to the circuit. With such a positioning all of the energizing circuits for test elements L will be opened. Therefore, photoconductive cells P will be subjected only to the radiation, including possible flame radiation, within the vehicle compartments under fire supervision and not filtered by window of the housing shown in Fig. 5. Window 120 is preferably in- 19 tor 93 due to the absence of substantial frequency components in the range of to 25 cycles per second.

In addition to the aforementioned characteristic of fire, the latching function of capacitor 95 also overcomes an additional characteristic which might possibly give a false fire termination indication. That is, while it has been stated generally that the band pass amplifier comprising tubes T1 and T3 will attenuate all components outside a selected range, it must be understood that from a practical aspect this attenuation at the upper and lower frequency limits is not sharp, but in actuality is a relative and gradual attenuation. In certain fires, there are periods during, which the frequency components within the range of l to 5 cycles per second are sufficiently intense in amplitude with respect to the components in the range of 5 to 25 cycles per second to create a fundamental square wave output'within the range of l to 5 cycles per second across load resistor 31 of limiter tube T4. Under these conditions, the components within the selected range will have no effect on the fundamental frequency of the output of the limiter because of the overriding action of the low-frequency components. The low-frequency square waves will be differentiated and transmitted to integrator capacitor 93 as hereinbefore explained, but a sufficiently large potential will not be integrated across capacitor 93 to break down tube T15. Therefore, during these intervals a false indication of the termination of a fire would occur, except that latching capacitor 95 draws sutficient probe current to maintain a charge which, together with the charge on capacitor 95, will successively break down tube T15, thereby avoiding the described false, indication of the termination of a fire.

The fire apparatus of Fig. 4 is preferably tested prior to the use of the aircraft in which the apparatus is installed, so that the pilot will know definitely that the apparatus is fully capable of performing its fire detection and supervision functions. In particular, test switch TS is successively positioned at the nine test positions before flight whereby the test lamps are successivelyenergized by current flow from secondary winding 112 through interrupter INT. Interrupter INT modulates the energy supplied to the test lamps at a frequency of approximately cycles per second, whereby the light emanating from the energized test lamps L is also modulated at a corresponding frequency. When the modulated light energy from a particular lamp L impinges upon its associated photoconductive cell P, a signal is transmitted throughout the circuitry in the same manner as that created by impinging flame radiation. Inasmuch as the fundamental frequency of this signal is 10 cycles per second, sufficient integration will occur in integrating capacitor 93 to break down gas tube T and thereby energize fire indicator FI.

If this procedure is repeated throughout all nine test positions, the proper operability of each of the fire detector pick-up units can be ascertained. If for any reason fire indicator F1 is not operated in response to the positioning of test switch TS at each of its test positions, repair and maintenance procedures should preferably be initiated before. operation of the aircraft.

A suggested installation of a commercial embodiment of the apparatus of Fig. 4 in a reciprocating engine aircraft isshown in Fig. 7. Fire detector units D are located within the engine nacelle so as to fully supervise both the power and the accessory sections thereof. These detecting units are individually cabled to an amplifier unit AU which houses the circuit apparatus. Amplifier unit AU is preferably located outside the aircraft Zones which are primarily subject to fire. Test switch TS is preferably located on the instrument panel for operative conven ience, and fire indicator F1 is also preferably located on the instrument panel so that the pilot may readily be informed of the occurrence of a fire or the results of a testing operation.

A suggested installation of the fire detection apparatus of Fig. 4 in a jet type aircraft is shown in Fig. 6. Generally speaking, the installation of the amplifier unit AU, test switch TS, and fire indicator F1 is the same as that hereinbefore described with respect to the reciprocating engine type of aircraft. Fire detector units D are disposed in the air space between the shroud and the skin of the aircraft so that any flame appearing outside the walls of the accessory section 126, compressor section 1.27, burner tubes 128, turbine 129, tail comb 130, and tail pipe 131 is immediately detected.

A third embodiment of this invention which is particularly adapted to supervise the continuance of intentional fires, is shown in Fig. 8 The type of output function provided by this circuit, generally speaking, is the reverse of that provided by the circuits of Figs. 1 and 4. That is, the circuit provides a null response during a tire and a control function in response to the extinguishment of a tire. For the sake of example, photoconductive cell 1 is shown coupled by pipe 132 to a fragmentary portion of the shroud assembly of the jet aircraft shown in Fig. 6, whereby the continued presence of a flame within burner 128 is supervised by the circuit oi Fig. 8. During the operation of the jet aircraft, flame should be continuously present within the burner. The apparatus shown in Fig. 8 can be utilized to immediately inform the operator of the unwanted extinguishment of such a flame or to initiate some particular control function in response to the extinguishment of such flame. It should be understood, however, that this embodiment of the invention is equally adaptable to the supervision of intentional fires within all types .of coinbustion chambers so that a change in output function will occur in response to the extinguishment of a flame.

Tubes T and T3 and their associated components comprise a band pass amplifier which is identical in structure to the band pass amplifier of the circuits of Figs. 1 and 4. The circuit components of Figs. 1 and 8 which perform corresponding functions are referenced by the same characters and a detailed description of the operation thereof may be had by reference to the dc scriptive portions of the circuit of Fig. 1.

The alternating-potential output of the band pass amplifier developed across load resistor 27 in response to the detection of fire flame by photoconductive cell 1 is coupled to the control grid G22-cathode C22 space path of triode T22 by serially-connected capacitor 134 and resistors 136 and 137. Diode T20 shorts to ground the positive alternations of thte alternating-current output developed across load resistor 27 whereby only a negative potential appears across resistors 136 and 137 with respect to ground. Capacitor 138 shunts resistor 137 so that a relatively smooth direct-current potential is applied to the control space path of tube T22 whenever an alternating-current signal is developed across resistor 27.

The anode-cathode space paths of tubes T21 and T22 are connected in series with respect to one another through resistor 139. An alternating-current potential is applied to the anode-cathode space path of tube T21 through the winding of relay RR by the portion of the secondary winding of transformer 14-6 between conductors 141 and 143. An alternating-current potential is applied to the anode-cathode space path of tube T21; through resistor 139 by the portion of the secondary winding of transformer 146 between conductors 143 and 144. Control grid G21 is connected directly to anode A22 so that the value of negative bias applied to the control space path of tube T21 is determined by the current flow in the anode-cathode space path of tube T22. in particular, if appreciable current flows in the anodecathode space path of tube T22, a sufficient voltage drop will be developed across resistor 139 to cut off the anode cathode space path current flow of tube T21 so that relay RR will release its contact, thereby completing the energizing circuit for output lamp LL. If, however, a sufliof half-wave rectifier tube T19. .also applied to the anodes A1 and As through their .respective load resistors 18 and 27 eien't negative potential is applied to the control space path of tube T22 to limit the voltage drop across resistor 139, tube T21 will conduct appreciable current in its anode-cathode space path and relay RR will be operated, thereby opening the energizing circuit for output lamp LL. Capacitor 140 shunts the winding of relay RR so that a relatively smooth operate current is applied to relay RR, thus preventing possible relay chatter.

Conductor 142 applies an alternating potential to the anode of half-wave rectifier tube T19 whereby a positive potential with respect to ground is developed at the cathode C19. Capacitor 135 filters this potential before the application thereof to the junction of resistors 133 and 27. Voltage regulator gas diode T18 and its limiting resistor 133 provide conventional filtering and voltage regulation action so that a relatively smooth and constant direct-current potential is applied to the junction of resistors 12 and 18.

The detailed operation of the circuit of Fig. 8 in responce to a supervised fire within burner 128 is as follows. When the appropriate primary potential is applied to winding 147 of transformer 146, the filaments F1, Fa, F19, F20 and F21 are heated in the conventional manner by winding 145. -An alternating-current potential is applied to the anode-cathode space path of tube T21 through the winding of relay R by the portions of the secondary winding of transformer 146 between conductors 141 and 143. An alternating-current potential is applied to the anode-cathode space path of tube T22 through resistor 139 by the portion of the secondary winding of transformer 146 between conductors 143 and 144. A positive potential is applied to photoconductive cell 1 through resistors 7, 12 and 133 from the cathode A positive potential is The fluctuating flame radiation within burner 128 causes :a corresponding fluctuation in the impedance of photoiconductive cell 1 whereby a complex alternating-current wave form having components principally in the band pass :range of the amplifier comprising tubes T1 and T3 is developed across load resistor 27. The positive alternations of this complex wave form are shorted to ground by V the low impedance presented thereto by diode T20. The

negative alternations of this complex wave form are not affected by the high reverse resistance of diode T20 and therefore a negative potential is developed across resistor 137 in a manner hereinbefore explained. This negative potential reduces the potential drop across resistor 139 so that appreciable current flow occurs in the anode-cathode space path of tube T21, thereby causing relay RR to operate. The operation of relay RR opens the energizing circuit for output lamp LL. Consequently, lamp LL is deenergized in response to the continued supervision of fire flame by photoconductive cell 1. If for any reason the-flame within burner 128 should become extinguished, the alternating-current output would not appear across load resistor 27 of the band pass amplifier, and, therefore, the negative bias developed during the supervision of flame across resistor 137 would disappear and tube T22 would conduct appreciable current, thereby applying a substantial negative bias to the control space path of tube T21. This negative bias reduces the current flow through the winding of relay RR to a value less than the relay release value. Consequently, relay RR releases its contact and the break contact thereon is closed, completing the energizing circuit for output lamp LL. The operation of output lamp LL therefore indicates that the flame within burner 128 has become extinguished. It should be understood that other control circuits may be operated by additional contacts on relay RR so as to provide any desired output function in response to the extinguishment ofa flame within a supervised chamber or in response-to the continued existence of said flame.

v As in the case of the embodiment of this invention short time interval.

22 shown in Fig. 4, the circuit of Fig. 8 is affected by the existence of flame periods during which the frequency components'within the selected range of the band pass amplifier are negligible in amplitude. During these periods the negative potential applied to the control space path of tube T22 will be diminished in amplitude because of the reduced output across resistor 27. This reduced potential will have a tendency to cut off the anode-cathode current flow in tube T21 so that relay RR will release its contact and close the energizing circuit for output lamp LL, thereby rendering a false flame termination indication. However, inasmuch as the release current value for conventional relays is much'less than the operate current value, an appreciable reduction in the current flow through the winding of relay RR is required before the relay will actually release its contact. This difference between the operate and release values for relay RR provides for a mode of operation which is analogous to the operation of latching capacitor of the circuit of 'Fig. 4. That is, notwithstanding an appreciable reduction in the I output signal across resistor 27 during the aforementioned periods, relay RR will remain operated because of its relatively low release current value. If a device having ditferent operate and release values is not utilized to control the operation of the output circuit, specific latching components should preferably be incorporated inthe modified arrangement.

The circuit of Fig. 8 is relatively simple in the number of componentsrequired to provide the desired functions because, generally speaking, the complex discriminator circuits shown in Figs. 1 and 4 are omitted. The discriminator circuits were necessary in the embodiments of Figs. 1 and 4 so as to fully assure that the fire apparatus will not operate in response to low-frequency lighting transients. Such assurance, however, is not necessary in the embodiment shown in Fig. 8 inasmuch as low-frequency transients, occurring after the supervised fire has been extinguished, will have only a momentary eifect upon the operation of relay RR, which effect can only open the energizing circuit for output lamp LL for a relatively After the transient has disappeared, and if no flame is being supervised, relay RR will again release its contact and permit the operation of output lamp LL. If it is desired to eliminate this momentary opening, a discriminator circuit should be added.

In view of the foregoing detailed structural and operational description, it should be obvious to anyone skilled in the art to which this invention pertains that the frequency response of the fire apparatus need not necessarily be limited to the range of 5 to 25 cycles per second. Generally speaking, with respect to the attainment of the objects of this invention, the lower-frequency limit need only be capable of attenuating the non-flame lowfrequency transients and/ or steady radiation components to which the apparatus will be actually subjected. This can be readily accomplished by a band pass amplifier or a discriminator circuit, or preferably by utilizing'both of these components as was done in the circuits of Fig. 1 and Fig. 4. Gther apparatus Will, of course, operate satisfactorily. Likewise, the upper frequency limit of the apparatus is defined by the non-flame high-frequency fluctuating components actually present in a particular installation. For a particular installation and purpose it might even be desirableto construct apparatus which is responsive only to components of a single modulating frequency.

It should also be understood that the novel contributions of this invention to the fire detection and supervision art is broader than that of the particular apparatus disclosed herein. From the broadest aspect, what is disclosed is a novel and basic method for producing fire responses, namely, the translation or" the intensity of thefluctuating radiation componentsof fire into a complex alternatingcurrent wave form of corresponding intensity and utilizing the frequency components of this wave =form.uriique to fire to produce an output function. This method may be practiced preferably by apparatus of the general construction shown in the drawings. However, it would also be possible to practice the novel method of this invention by utilizing non-equivalent apparatus. For example, a digital computing arrangement could be utilized to count modulated radiation components within a supervised area, and if the count for a certain time interval was within the unique range for fire flame, the presence of fire would be indicated.

What is claimed is:

1. In combination, means for detecting the presence of fire flame within a supervised volume, means for developing an output in response to detected modulated components of said fire flame within specified upper and lower modulation frequency limits in the audio to subaudio range, and means for operating an alarm in response to the output of said second means.

2. In combination, means for detecting fire flame, means for developing an electrical potential whose amplitude substantially corresponds to the intensity of the flame radiant energy received by said detecting means, and means responsive to specified audio and sub-audio frequency components of said potential within a relatively narrow range.

3. In combination, means for detecting radiant energy emanating from fire flame, means for developing an electrical potential whose amplitude is determined by the fluctuating intensity of the radiant energy detected by said first means, means for discriminating against the alternating-current components of said electrical potential having sub-audio frequencies equal to or less than the maximum frequency of the intensity of the transient lighting to which said detecting means is subjected, means for discriminating against the alternating-current components of said electrical potential having frequencies within the audio and sub-audio range equal to or greater than the lowest frequency of the intensity of the artificial lighting to which said detecting means is subjected, and means responsive to the alternating-current components of said electrical potential having frequencies between said upper and lower discrimination limits.

4. In combination, means for detecting the radiant energy from fire flame, means for developing an electrical potential whose amplitude is determined by the intensity of the radiant energy detected by said first means, means for producing an output determined by the alternatingcurrent frequency components of said potential within specified lower and upper frequency limits in an audio and sub-audio range when measured over a specified time limit, and means responsive to said transmitted alternating-current potential components.

5. The combination of claim 4 wherein said specified time limit is one second and said lower and upper frequency limits are approximately five and at least ten cycles per second, respectively.

6. Fire detection apparatus, comprising'one or more light-responsive elements, means for translating the fluctuating light energy impinging upon said elements from fire to fluctuating electrical currents, means for generating a specified operative output potential in response to a selected band of current components of said translating means within the audio and sub-audio frequency range, said current components being generated by the low frequency flickering of the fire flame which impinges upon one or more of said light-responsive elements, and means for actuating a fire alarm in response to the generation of said output potential.

7. The combination of claim 6 including a trouble alarm, means for transmitting a trouble-supervising signal throughout said light-responsive elements, translating means, potential generating means and fire alarm actuating means, and means for actuating said trouble alarm in response to the incomplete transmission of said troublesupervising signal.

8. Fire detection apparatus, comprising one or more light-responsive elements, means for generating an electrical output determined by the amplitude of .the light impingingupon one or more of said light-responsive elements, a band pass amplifier with limits in the audio and sub-audio range connected to said generating means and being selectively responsive to a' portion of the alternating current components of the electrical output generated by the fluctuating light from fire fiame impinging upon one or more of said fire-detecting elements, and means for operating a fire alarm in response to the output of said band pass amplifier.

9. The combination of claim 8 including a trouble alarm, means for transmitting a trouble supervising signal throughout said light-responsive elements, band pass amplifier and fire alarm operating means, and means for operating said trouble alarm in response to the interrupted transmission of said trouble-supervising signal.

10. Fire-responsive apparatus, comprising one or more light-responsive elements, means for generating an electrical output determined by the amplitude of the light impinging upon one or more of said light-responsive elements, a band pass amplifier connected to said generating means and being selectively responsive to specified frequencies of the alternating-current components of the electrical output generated through light fluctuations of fire flame impinging upon one or more of said fire-detecting elements, fire apparatus, and means including a discrimination network for operating said fire apparatus when the output of said band pass amplifier continues for a specified time interval.

11. The combination of claim 10 including a trouble alarm, means for transmitting a trouble-supervisory signal throughout said light-responsive elements, band pass amplifier and means for operating the fire apparatus, and means for operating said trouble alarm when the transmission of said trouble-supervising signal is interrupted.

l2. Fire-responsive apparatus, comprising means for generating an amplitude-modulated electrical output responsive to the fluctuating intensity of the radiant energy emanating from fire flame, means for selectively transmitting specified low-frequency components of said elec trical output within the audio and sub-audio range, means for discriminating against said selectively transmitted components unless a given minimum number thereof are transmitted within a specified time interval, fire apparatus, and means for actuating said fire apparatus when the selectively transmitted components exceed said minimum value.

13. The combination of claim 12 including trouble means, means for transmitting a supervising signal throughout a portion of said fire-responsive apparatus, and means for operating said trouble means when the complete transmission of said supervising signal is interrupted.

14. Fire detection apparatus, comprising one or more photosensitive cells, means for energizing said cells whereby an electrical output is generated dependent upon the intensity of the radiant energy emanated by fire flame within a fire-supervised volume, means for amplifying the alternating-current frequency components of said electrical output generated by fire amplitude fluctuations within a frequency range having a lower limit of approximately five cycles per second and an upper limit of not less than ten cycles per second, means responsive to the occurrence within a time interval of one second of a specified number of electrical impulses from said amplifying means, and output means operated by said responsive means.

15. Fire apparatus comprising a plurality of photoresponsive networks connected in parallel with respect to one another so as to generate individual output signals in response to the intensity of the radiant energy impinging upon each network, a band pass amplifier selectively transmitting a range of frequency components unique to fire flame, a mixing network interconnecting said plurality of photoresponsive networks and the input of said band pass amplifier whereby the input signal applied to said band pass amplifier is responsive to the output of all of said photoresponsive networks, an output network, a'discriminating network interconnecting said output network and said band pass amplifier whereby said output network is operated only in response to the continued detection of fire flame by one or more of said photoresponsive elements for greater than aspecified time interval, and a latching network for maintaining said output network operated during the fire periods wherein the amplitude of said selectively transmitted frequency components would be otherwise incapable of operating said output network.

16. The combination of claim including a plurality of test lamps individually coupled to difierent ones of said photoresponsive networks, an energy source modulated at a frequency within the range of said band pass amplifier, and a test switch capable of individually interconnecting said test lamps and said modulated energy source whereby a fire can be simulated by said lamps so as to test the operability of the fire apparatus.

17. Fire apparatus comprising a plurality of photoelectric fire detection units, a band pass amplifier selectively responsive to a frequency range of approximately five to twenty-five cycles per second, a signal mixing network interconnecting said photoelectric cells and said band pass amplifier so as to combine the individual outputs of said photoelectric cells into a single resulting signal which is applied to the input of said band pass amplifier, a limiter network connected to the output of said band pass amplifier and translating the output of said amplifier to signals having a square wave form, a differentiating network connected to said limiter network and translating said square wave form signals into sharply peaked signals, an integrating network for generating a potential the magnitude of which is determined by the rate at which specified impulses of said sharply peaked signals are applied thereto, an output network connected to said integrating network and being operated whenever said generated potential exceeds a specified value, and a latching network connected to said output network and maintaining said output network operated during the fire periods wherein the amplitude of said selectively amplified frequencies is insufiicient to maintain the potential generated by said integrating network at a value greater than said specified value.

18. The combination of claim 17 including a plurality of test lamps individually coupled to different ones of said photoelectric fire detection units, an energy source modulated at a frequency within the range of said band pass amplifier, and switching means capable of individually interconnecting said test lamps and said modulated energy source whereby a fire can be simulated by said lamps so as to test the operability of the fire apparatus.

19. Fire apparatus for aircraft comprising a plurality of photoelectric fire detection units positioned within the compartments of said aircraft primarily subject to fire, a band pass amplifier for selectively amplifying a range of frequencies generated by said'detection units within specified upper and lower limits, the values of said upper and lower limits being determined so that said amplifier will attenuate the modulating frequencies of the nonflame radiation components to which said photoelectric detection units are responsive, and an output network operated in response to the output of said band pass amplifier.

20. The combination of claim 19 including latching means for maintaining said output network operated dur- 26 ing the fire periods wherein the amplitude of said selectively amplified frequency components would otherwise be incapable of operating said output network.

21. The combination of claim 19 including a plurality of test lamps individually coupled to different ones of said photoelectric fire detection units, an energy source modulated at a frequency within the range of said band pass amplifier, and switching means capable of individually interconnecting said test lamps and said modulated energy source whereby a fire can be simulated by said lamps so as to test the operability of the fire apparatus.

22. Fire apparatus comprising a photoelectric cell optically coupled to a combustion chamber so as to supervise the fire flame therein, a band pass amplifier for selectively amplifying the currents generated by said photoelectric cell within a range of frequencies having specified upper and lower limits, the values of said upper and lower limits being determined so that said amplifier will attenuate the frequency components generated by non-flame radiationimpinging upon said photoelectric cell, means for translating the output of said band pass amplifier into a relatively smooth direct-current potential, and means for operating a relaycontrolled output circuit in response to the intensity of said direct-current potential.

23. In combination, means for detecting the radiant energy from fire flame, means for developing an electrical potential whose amplitude is determined by the intensity of the radiant energy detected by said first means, means for producing an output determined by the alternatingcurrent frequency components of said potential within specified lower and upper frequency limits in an audio and sub-audio range when measured over a specified time limit, and means responsiveto said transmitted alternatingcurrent potential components, said specified time limit being one second and said lower and upper frequency limits being approximately five and twenty-five cycles per second, respectively.

24. Fire-responsive apparatus, comprising means for generating an amplitude-modulated electrical output responsive to the fluctuating intensity of the radiant energy emanating from fire flame, means for selectively transmitting specified low-frequency components of said electrical output within the audio and sub-audio range, means for discriminating against said selectively transmitted components unless a given minimum number thereof are transmitted within a specified time interval, ,fire apparatus, and means for actuating said fire apparatus when the selectively transmitted components exceed said minimum value, the frequency of said specified components being within a range of approximately five to twenty-five cycles per second.

25. The combination of claim 1 wherein said upper and 7 lower modulation frequency limits are approximately twenty-five and five cycles per second respectively.

26. The combination of claim 2 wherein said lastnamed means comprise a band-pass amplifier tuned to a frequency between five and twenty-five cycles per second.

27. The combination of claim 22 wherein said bandpass amplifier is tuned to a frequency between five and twenty-five cycles per second.

References Cited in the file of this patent UNITED STATES PATENTS Dauvillier Oct. 3, 1950 

